NASA Technical Reports Server (NTRS) 19990009102: Characterization of Aircraft Produced Soot and Contrails Near the Tropopause

22 Pages·0.97 MB·English

by  NASA Technical Reports Server (NTRS)

1.Characterization of Aircraft Produced Soot and Contrails near the Tropopause
2. Summary of Research
3.John Hallett; James G. Hudson
4. 6/1/94 to 12/31/97
5.Desert Research Institute, PO Box 60220, Reno, NV 89506
6. NAG 2- 923
I. Introduction.
Participation in the SUCCESS project primarily involved development and
deployment of specific instruments for characterizing jet aircraft exhaust emissions
as particulates and their subsequent evolution as contrail particles, either liquid or
solid, as cirrus. Observations can be conveniently considered in two categories -
close or distant from the aircraft. Thus close to the aircraft the exhaust is mixing
through the engine turbulence with a much drier and colder environment and
developing water/ ice supersaturation along the trail depending on circumstances
(near field), whereas distant from the aircraft (far field) the exhaust has cooled
essentially to ambient temperature, the turbulence has decayed and any particle
growth or evaporation is controlled by the prevailing ambient conditions.
Intermediate between these two regions the main aircraft vortices form (one on
each side of the aircraft) which tend to inhibit mixing under some conditions, a
region extending from a few aircraft lengths to sometimes a hundred times this
distance. Our approach to the problem lay in experience gained in characterizing the
smoke from hydrocarbon combustion in terms of its cloud forming properties and
its potential influence on the radiation properties of the smoke and subsequent
cloud from the viewpoint of reduction (absorbtion and scattering ) of solar radiation
flux leading to significant global cooling (Hudson et al 1991; Hallett and Hudson
1991). Engine exhaust contains a much smaller proportion of the fuel carbon than
is sometimes present in ordinary combustion (<0.01% compared with 10%) and
influences condensation in quite different ways, to be characterized by the Cloud
Condensation Nucleus, CCN - supersaturation spectrum. The transition to ice is to
be related to the dilution of solution droplets to freeze by homogeneous nucleation
at temperatures somewhat below -40C (Pueschel et al 1998). The subsequent growth
of ice particles depends critically on temperature, supersaturation and to some
extent pressure, as is demonstrated in an NSF funded project being carried out in
parallel with the work reported here. As will be discussed below, nucleation
processes themselves and also exhaust impurities also influence the growth of ice
particles and may control some aspects of growth of ice in contrails. Instrumentation
was designed to give insight into these questions and to be flown on the NASA DC-
8 as a platform. In addition a modest program was undertaken to investigate the
properties of laboratory produced smoke produced under controlled conditions
from the viewpoint of forming both CCN and CN. The composition of the smoke
could inferred from a thermal characterization technique; larger particles were
captured by formvar replicator for detailed analysis; ice particles were captured and
evaporated in flight on a new instrument, the cloudscope, to give their mass,
density and impurity content.
SEP1 4 lgg8
II Scientific Questions.
We examine the scientific questions which we perceived as driving our
participation in the project. The fundamental mechanism to be probed is whether
and how particles emitted from engine exhausts lead to contrail formation under
different conditions in the atmosphere. A further question is whether such particles
can (prior to removal by various scavenging process) be recycled to form much more
extensive upper tropospheric cloud as increased area coverage or thicker (greater
optical depth) cloud. Such events could lead to a change in the earth's radiation
balance resulting in changes in global temperature and precipitation distribution. A
parallel question is related to fuel composition and whether impurity removal or
additives could change the situation - or whether indeed modest changes in engine
design could be undertaken. Such considerations lead to questions on particle
nucleation, particle growth and evaporation from a trajectory from engine exhaust
to how ultimately the particles are scavenged out by precipitation. Specific
instrumentation has been designed, built and used to answer aspects of these
questions. It is to be pointed out here that particles not only interfere with radiation
but also act as substrates for catalyzing chemical reactions so that our findings may
have relevance to ozone chemistry particularly at low temperature (Carslaw et al
1998).
III Results.
Summarized below are the general results that follow from the listed
publications in section V and publications in preparation:
* Aircraft measurements of aircraft exhaust CCN (0.2 -1%) and CCN/CN ratio at
contrail levels showed substantially higher values than had been measured at lower
levels in the atmosphere. A possible explanation lies in the presence of very clean
air, as a product of previously existing cirrus, leading to an effective scavenging of
CCN as they dilute, nucleate ice and are removed by ice precipitation to somewhat
lower levels where ice evaporation occurs (Figure 1). Thus ice precipitation and the
presence of very high values of CN is an observation consistent with fresh
nucleation in the absence of CCN to provide a basis for chemical reactions (Hudson
and Xie, 1998).
* These results together suggest that although the high and low sulfur fuel may
give rise to exhaust with high and low vales of CCN, there may be somewhat of a
feedback process whereby ultimately natural ice processes may give a dearth of CCN
which in turn leads to high CN nucleation and more CCN following aggregation.
The net result in either event would be some increase in cirrus cloud presence (area)
and/or optical depth with the presence of more sulfur in the exhaust. These
considerations show that we need to obtain better insight into the overall aerosol
particle economy of clouds and their environments at these levels. (In preparation).
* Measurements of CCN were made of smoke produced from high and low
sulfur fuel in a laboratory test burn. In both cases CCN for high sulfur fuel was
substantially (X 10) above that for low sulfur fuel suggesting an increased
hygroscpic component of the particles; heating the high sulfur sample to 180C
reduced the CCN concentration in the smoke by about X 20, whereas heating had a
minimal effect on smoke from low sulfur smoke, (Hallett et al 1997).
* Particles collected on replica in an evaporating contrail were small (10
microns) , had excessive trigonal symmetry, (suggesting a defect/impurity effect)
and concentrations up-to 1 per ml, (figures 2, 3,4). The variability appeared greater
for flight segments from low sulfur fuel. Such particles were not collected on the
cloudscope, suggesting a density of less than 0.2 and a low collection efficiency
(Meyers & Hallett, 1998; Meyers, Arnott and Hallett in preparation). This low density
is consistent with rapid evaporation rate of similar larger ice particles observed on
the cloudscope under other circumstances. It appears that a higher proportion of
crystals with trigonal symmetry was associated with higher sulfur fuel burn.
* Cloudscope collection of droplets in a wave cloud over Boulder showed a
droplet concentration of somewhat less than 10 per ml of size some 15 microns.
These numbers do not differ substantially from the concentration of ice particles
(Jensen et al 1998). Comparison with CCN measurements suggested that only a very
small fraction of nuclei were activated requiring a supersaturation of below 0.1 per
cent over water. As these droplets evaporated, no visible residue remained,
consistent with the solid residue being well below one micron in diameter, the limit
of resolution, irrespective of its composition. (Hudson and Hallett in preparation).
* Accretion and evaporation of ice particles on the cloudscope at low
temperature (<-55C) showed, on occasion, the apparent melting of the ice as
evaporation progressed. The observation is consistent with the presence of a H2SO4
residue and dissolution of the ice as the solution concentration increases. This we
consider as an important observation, but is very tedious in analysis and is still
under study.
IV Instrumentation.
Existing instruments were adapted for installation on the DC-8 and modified for the
specific task of looking at the nature of particles in aircraft contrails. Instrument
location and details of the missions are shown in table 1.
A. The cloudscope.
The cloudscope was first constructed as an operating system in 1995 (Arnott et al
1995) and has been developed into a highly reliable instrument for aircraft
deployment. The operating principle is collection of particles near the stagnation
point on a forward facing optical flat, video recorded from behind with a resolution
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of about 2 microns. Collection falls for sizes of particle below some 10 microns such
that 2 micron diameter particles are not collected (Figure 5). The rate of change of
particle area following evaporation in the compressed air is to be related to particle
density (Hallett et al 1998). The instrument fits a PMS pod and is mounted under
the wing tip of the DC-8. Modifications were made for the SUCCESS mission to
improve illumination of the particles and the ease of mounting in the pod so that
instruments could be exchanged as required. Data analysis ( Arnott et al 1998;
Hallett et al 1998) uses a frame grabbing technique to capture the video images and
estimates their area differences during evaporation. The spatial concentration is
estimated from a knowledge of particle density, airspeed and the dimension of the
collecting housing. Shape (habit) is estimated directly from the image. It is usually
possible to obtain some estimate of hygroscopicity of such particles by observing
their behavior on the cloudscope under conditions of increasing relative humidity.
* The Replicator.
The principle of operation is to collect particles in a thin film of solution of formvar
on a moving 70 m length of 16mm film. The solution dries by evaporation of the
solvent to leave a cast of the ice or water particle, to be examined by microscopy in
the laboratory ( Hallett 1976; Arnott et al 1998). The version used in the DC°8 fits a
PMS pod and was, for this deployment, ventilated to give a particle collection
efficiency down to about 1 micron. In practice this resolution was a few microns
because of impurity particles in the solution. The film speed could be changed from
1/2 or 1 cm per second as required to give a sample time of up to 3 hrs.
Modifications were made for this mission to improve the speed of deployment prior
to each flight and in the viewing necessary to give better quality control during
flight. Reduced data are in the form of size, habit and phase distributions, together
with detailed images by optical and scanning electron microscopy.
C. The CCN Spectrometer.
The principle of operation is to use a vertical water vapor thermal diffusion
chamber to produce a gradient of supersaturation through which outside air passes
in a sheath flow allowing activation of aerosol which grow and are sensed optically
as they leave the chamber (Hudson 1989). The instrument is mounted in the cabin
and samples air from a by pass flow as required. The aerosol either passes to the
chamber directly or through a temperature processor operating up to a selected
temperature of 500C. The chamber operates at ambient pressure and concentration
reduced to a standard pressure for inter comparison. A supersaturation spectrum
requires about 5 seconds and is plotted as a differential concentration -
supersaturation; the spectrum may be extrapolated to higher/ lower supersaturation
as required thin limits of accuracy given by the spectrum variability (Figure 6). CCN
results were compared with CN data obtained by P.Demott/D.Rogers of CSU.
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V Aircraft Instrument Comparison.
During the course of this work and previous related projects, it has become
necessary to collect data from different instruments measuring different quantities
which are not co-located (Figure 7). Instruments are mounted on each wing tip and
also at various locations on the fuselage and such data need to be synthesized in
order to compute derived quantities, such as aerosol flux as a product of aerosol
number concentration (or area or mass) and vertical velocity for example. A similar
synthesis is involved in combining two instruments measuring different size
ranges in a particle spectrum - as small ice particles tens of microns and larger ice
particles many hundreds of microns. In as far as there is an approximate
exponential relationship through an empirical gamma distribution or a theoretical
relation ship through a Weibull distribution (Liu and Hallett 1998), there tends to be
a smaller concentration of larger particles. It follows that at some point for any
particle counting instrument a choice has to be made in assessing particles from the
viewpoint of Poisson statistics as the sample volume (or time) gives a small number
of particles in the sample interval, it being remembered that a concentration N per
sampled volume in some time interval t has a variance in a uniform field of N 1/2.
Thus assuming that instruments on each wing tip 'see' air with the same uniform
property, comparison, as a test for uniformity can only be achieved within these
limits. In a true uniform field with random distribution the sample time can, in
principle, be extended indefinitely; in a finite field limitations always exist. A casual
window view of the spatial distribution of particles in a contrail sampling mission
dramatically brings home this point - sometimes only one wing tip lies in the
contrail. And this is not all - when the contrail itself is wrapped into the aircraft
wing tip vortex, spatial variability exists as particle free contrail air is interleaved
with particle containing contrail air. Inhogeneities down to a scale of meters and
below readily exist. Differing airflow at the site of collection resulting from aircraft
flight changes present an additional difficulty.
A conclusion from these considerations is that while data from a single
instrument yields useful results over its (limited) dynamic range, combination of a
spectrum from two instruments differently located has to be treated with caution.
The same argument holds for comparison with integrated water, as with an ice mass
content instrument as a CVI. These thoughts are common to all aircraft
measurements and are dominating attempts at instrument comparison in
numerous cases in the SUCCESS mission. The problem is particularly worrying for
contrail studies as the intrinsic spatial scales are necessarily short. For larger scales
(as natural cirrus) intrinsic variability for two instruments need to be derived and
any comparison made on this basis. A paper is in preparation on this topic.
VI Related Project:
In parallel with this NASA study, a NSF funded laboratory study has been
undertaken of the habit of ice crystals growing from the vapor under cirrus
conditions. Specifically a water vapor thermal diffusion chamber has been
constructed which enables temperature, supersaturation over ice and air pressure to
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be controlled independently. The temperature range covers -30C to below -70C,
supersaturation from a few% to over 200% over ice, and air pressure from ambient
850 to 1 rob. Ice crystals are grown on a narrow vertical filament and the growth
rates and habits video recorded. Such habits can be mapped and related to cirrus
conditions as in Figure 8. The effective supersaturation is estimated from the
ventilation coefficient of a crystal falling at terminal velocity. (Bailey and HaUett,
1998). Future studies will involve addition of known impurities to the chamber
atmosphere to provide a series of maps similar to Figure 8.
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VII References.
Arnott,
W.P., Y. Dong, R. Purcell, and J. Hallett, 1995. Direct Airborne Sampling of Small Ice
Crystals and the Concentration and Phase of Haze Particles. 9_ Symposium on
Meteorological Observations and Instrumentation, Charlotte, NC, 27-31 March 1995, pp.
415-420.
Arnott,W.P., T.J. Ulrich, R.Cole and J.Hailett: 1998; Summary of in-situ observations of Cirrus
Microphysics During FIRE II: Instrumental Response. Conference on Cloud Physics,
Everett, WA, August 1998.
Bailey, M. and J. HaileR, 1998; Laboratory investigation ice growth from the vapor under cirrus
conditions between -30C and -70C, Conference on Cloud Physics, Everett, WA, August
1998, pp.434-437.
Carslaw, K.S., M. Wirth, A. Tsias, B.P. Luo, A. Dornbrack, M. Leutbecher, H. Volkert,
W. Renger, J.T. Bacmeister, E. Reimer and Th. Peter, 1998. Increased
stratospheric ozone depletion due to mountain-induced atmospheric waves. Nature, 391,
12 February, pp.675-678.
HaileR, J., 1976. Measurement Size Concentration and Structure of Atmospheric Particulates by
the Airborne Continuous Replicator, Final Report. AFGL-TR-76-1149.
HaileR, J. and J.G. Hudson, 1991. Stratospheric Soot: Cloud Forming Properties and
Transport Processes. AIAA/AHS/ASEE Aircraft Design Systems and Operations
Meeting, AIAA 9_hApplied Aerodynamics Conference, Baltimore, MD, 23-25
September, 1991.
Hudson, J. G., 1989. An Instantaneous CCN spectrometer. J. Atmos. Oceanic Technol., 6,
pp.1055-1065.
Hudson, J.G.J. HaileR, and C.F. Rogers, 1991. Field and Laboratory Measurements of Cloud-
Forming Properties of Combustion Aerosols. J of Geophys. Res., 96:D6, pp.10,847-
10,859, 20 June, 1991.
Jensen, E.J., O.B. Toon, A. Tabazadeh, G.W. Sachse, B.E. Anderson, K.R. Chan, C.W. Twohy,
B. Gandrud, S.M. Aulenbach, A. Heymsfield, J. HaileR, and B. Gary, 1998. Paper No.
98GL00299, lee nucleation processes in upper tropospheric wave-clouds observed during
SUCCESS. Geophys. Res. Lea., 25, pp.1363-1366.
Liu, Y. and J. HaileR, 1997. On Size Distributions of Cloud Droplets Growing by Condensation:
A New Conceptual Model. Am. Meteor. Sot., 55, pp.527-536.
Meyers, M. and J. Hailer 1998 Ice crystal Morphology in Aircraftcontrails and Cirrus,
Conference on Cloud Physics, Everett, WA, August 1998, pp. 17-20.
Pueschel, R.F., S. Verma, G.V. Ferry, S.D. Howard, S. Vay, S.A. Kinne, J. Goodman, and A.W.
Strawa, 1998. Paper No. 97GL03479, Sulfuric acid and soot particle formation in aircraft
exhaust. Geophys. Res. Lett. 25:10, pp.1685-1688, 15May, 1998.
Publications resulting directly* from or related t to this grant:
tHallett, J., J.G. Hudson and R. Pueschel 1996 Aerosol in Tropical Cirrus, In 14th
Conference on Nucleation and Atmospheric Aerosols, Ed. Kulmala and Wagner,
Helsinki, pp.880 - 883.
*Hallett, J., J.G. Hudson, F. Rogers, E. Teets, S.S. Yum and Y. Xie, 1997, The Influence
of Natural and Aircraft Exhaust on Aerosol and Cirrus Formation. Third
Conference on Atmospheric Chemistry, AMS, February 1997, Long Beach CA
pp.45 - 53.
*HaileR, J., W.P. Amott, R. Purcell and C.S Schmidt: 1998, A Technique for
Characterizing Aerosol and Cloud Particles by Real Time Processing. In
Proceedings, PM2.5: A Fine Particle Standard Conference, Long Beach CA, 28-
30 January 1998, pp.318 - 325.
*Hudson, J.G. and Y. Xie., 1997. Cloud condensation nuclei measurements in the high
troposphere and in jet aircratt exhaust. Geophys. Res. Lett., 25, pp. 1390-1395.
*Jensen, E.J., O.B. Toon, A. Tabazadeh, G.W. Sachse, B.E. Anderson, K.R. Chan, C.W.
Twohy, B. Gandrud, S.M. Aulenbach, A. Heymsfield, J. Hallett, and B. Gary,
1998. Paper No. 98GL00299, Ice nucleation processes in upper tropospheric
wave-clouds observed during SUCCESS. Geophys. Res. Lett., 25, pp.1363-1366.
*Poellot, M.R., W.P. Amott and J. Hallett, 1998, In Situ Observations of Contrail
Microphysics and Implications for their Radiative Budget. J.G.R., In Press.
Papers presented at the AMS Cloud Physics Conference, Everett WA August 1998:
tArnott, W.P., T.J. Ulrich, R.Cole and J.Hallett: 1998; Summary of in-situ observations
of Cirrus Microphysics During FIRE II: Instrumental Response. Conference on
Cloud Physics, Everett, WA, August 1998.
*Bailey, M. and J. Hallett, 1998; Laboratory investigation ice growth from the vapor
under cirrus conditions between -30C and -70C, Conference on Cloud Physics,
Everett, WA, August 1998, pp.434-437.
*Meyers, M. and J. Hallett 1998 Ice crystal Morphology in Aircraft contrails and Cirrus,
Conference on Cloud Physics, Everett, WA, August 1998, pp, 17-20.
tSchmitt,C., W.P. Arnott and J. Hallett,1998; Infrared Extinction and Emission by
Laboratory Ice Clouds: Simulation of Cold Cirrus and Contrail Radiative
Properties. Conference on Cloud Physics, Everett, WA, August 1998.
Presentations to be made at the CIRRUS Conference in Baltimore, October 1998:
*Shape and Fall of Cirrus Particles.
(Invited paper) John Hallett and Pat Amott (DR/).
*New Insight for in situ quantification of cirrus
W. Patrick Amott and John Hallett (DR/) and Michael R. Poellot (UND)
Papers in Preparation:
*Laboratory study of cirrus growth habit related to environmental parameters.
Matthew Bailey and John Hallett
*Kinne, S., J. Halle*t, et al
Intercomparison of aircraft measurements by instruments mounted at different
locations.
*Hallett,J., J.G.Hudson, F.Rogers, E.Teets, S.S. Yum and Y. Xie, The Influence of
Natural and Aircraft Exhaust on Aerosol and Cirrus Formation.
VIII Figures
SUCCESS
April 27, 1996
I000
I i I I I I I I [ I t I I I I ! i
--*-- Above
---4-- Clean
--*-- Below
'E I00
0
r_
0
m
a= 1o
L_
I
0.01 0.I 1
s(%)
Figure 1. CCN spectra at contrail levels showing high variability in and out of
old contrail regions.